The invention relates generally to image projector systems and more particularly to registration of the images projected from each image projector of the system.
In some systems, an image projected on a screen must have a particular register, i.e., with a reference point or with another image or images projected on the same screen. As an example, where the images of two cathode ray tubes (CRT), each CRT image having a difference color, are to be projected onto a single screen to form a single image having the color combination of both CRT images, the screen registration accuracy requirement for the two images can be very severe in order to avoid undesirable color fringing. The registration accuracy requirement may require registration within one picture element. In some systems where high resolution is required, a registration accuracy requirement of registration within one-fourth of a picture element may exist. For a CRT picture which is four inches in length in one dimension and has one thousand picture elements on that axis, a registration requirement of registration within 0.001 inch may exist.
Since image registration has been affected by component drift due to heat, warm-up times, age, use, etc., some prior techniques invoked sophisticated designs of electrical, optical and mechanical components for stability in view of these factors. Even with these techniques, component drift was not compensated for and registration needed to be readjusted frequently. A further technique was the application of thermostatically controlled heaters to bring all components to a selected temperature which above the ambient temperature. This technique still did not provide the registration accuracy required and the heat had an adverse effect on component reliability.
U.S. Pat. No. 4,683,467, assigned to a common assignee as the present application, discloses an image registration system for registering multiple images on a screen. A registration pattern is projected on the screen along with the image to be registered and in a fixed position in that image. The sensor apparatus for each image, having three photodetectors disposed in a right angle relationship is mounted on the screen at a predetermined location for determining the position of the registration pattern on the screen. The position of the registration pattern on the screen is determined by comparing the outputs of the three photodetectors. Difference signals are produced by the comparisons and are used to reposition the projected image to bring it into registration. The sensor apparatus are disposed so that when the registration patterns are in register with them, the images are in register with each other.
While the system of U.S. Pat. No. 4,683,467 provides a substantial advance over the aforementioned image registration techniques, a disadvantage is the large size of the photodetector assembly required, and the installation problems associated with its mounting and alignment.